Run V8 with
--prof --log-timer-events,
or alternatively,
Chrome with
--no-sandbox --js-flags="--prof --log-timer-events"
to produce v8.log.

Open
v8.log
on this page. Don't worry, it won't be uploaded anywhere.

Click "Start" to start number crunching. This will take a while.

Click "Show plot/profile" to switch between the statistical profile and
the timeline plot.
C++ items are missing in the statistical profile because symbol
information is not available.
Consider using the
command-line utility
instead.

If you expect multiple V8 instances to run concurrently, for example
with several tabs in Chrome,
add the V8 flag --logfile=v8.%p.log
so that each instance writes to its own log file.